1. Field of the Invention
The invention relates to a radial runout detection method and device and, more particularly, to a radial runout detection method and device for an optical disk drive.
2. Description of Related Art
With the advance of information technologies, optical disks capable of storing a large quantity of data have become an essential storage in a computer system. Accordingly, optical disk drives are now standard equipment in a computer to read and write the optical disks. FIG. 1 shows a schematic view of an optical disk 11 with stored information. As shown in FIG. 1, the surface on the optical disk 11 has multiple data tracks 112 centering the disk center 111 in a substantially concentric arrangement, thereby recording the information. FIG. 2 shows a schematic view of an optical disk drive reading the information on the optical disk 11. As shown in FIG. 2, the optical disk 11 is placed on a turntable 14 and driven by a spindle motor 12 to rotate, and a movable optical pickup unit 13 generates a laser beam. The laser beam generates a beam spot on the surface of the optical disk 11. The beam spot is reflected to thus read the information on the optical disk 11. Ideally, when the optical disk 11 is rotated by the spindle motor 12, a beam locus 31 shaped by the beam spot generated by the optical pickup unit 13 falls just on a data track 112 to accordingly read the information on the optical disk 11. However, in practice, as shown in FIG. 3, due to poor manufacture of the optical disk 11, slanting of the turntable 14 and displacement of the spindle motor 12 may cause the center 111 of the data track 112 of the optical disk 11 and a center 111′ at rotation to fail to locate at the same point, which further causes so-called radial runout. Thus, the beam locus 31 shaped by the beam spot and the data track 112 are displaced from one another.
In order to detect the magnitude of the radial runout and accordingly correct the optical disk drive to accurately read the information of the optical disk 11, a typical approach is to use a tracking error (TE) to compute a track-crossing number that indicates the number of data tracks 112 crossed by the beam locus on the optical disk 11 during a period in which a predetermined number of revolutions is performed. FIG. 4A shows a schematic view of a data track 112 crossed by a beam locus 31 for one revolution. As shown in FIG. 4A, the runout track number (radial runout measured in tracks) is about a half of the number of data tracks 112 crossed by the beam locus 31 as the optical disk 11 rotates for one revolution. Accordingly, FIG. 4B is a flowchart of a radial runout computation. As shown in FIG. 4B, when the optical disk 11 rotates for a predetermined number of revolutions, the number of data tracks 112 crossed by the beam locus 31 is computed to thus find the runout track by dividing the track-crossing number by two times the predetermined number of revolutions, thereby obtaining the radial runout.
However, such a radial runout detection needs to know the predetermined number revolutions. In this case, the spindle motor used is a three-phase AC motor equipped with a Hall sensor to generate a FG signal with respect to the rotation frequency. Thus, the number of revolutions is obtained. However, the three-phase motor and its equipped driver are expensive. In order to reduce the cost, a typical low-speed optical disk system uses typical single-phase DC motors as the spindle motors. In this case, there is no FG signal as a reference so that the predetermined number of revolutions cannot be obtained and the radial runout cannot be detected by the cited technique.
U.S. Patent Application Publication No. 20050152446 entitled “Method for detecting eccentricity of an optical disc, and optical disc drive that performs the method” is provided to detect a radial runout (eccentricity) of a testing disk on an optical disk drive, which controls the spindle motor of the optical disk drive at a predetermined rotation speed when the optical disk drive is on a focused and track-locked state, and performs a runout detection on the testing disk, thereby obtaining a runout value. The runout value obtained is further compared with a reference to determine a radial runout of the testing disk. The reference is a predetermined runout value obtained by previously testing a standard radial runout disk. Such a radial runout detection is carried out after the focusing and track-locking operations are complete. However, due to the radial runout of the tracks, a track-locking failure may occur, resulting in an unpractical detection of the radial runout.
Therefore, it is desirable to provide an improved radial runout detection method and device for an optical disk drive to mitigate and/or obviate the aforementioned problems.